Ink-jet printer

ABSTRACT

An ink jet printer has two first driven rollers (A) disposed at a most upstream position in a secondary scanning direction, two first driven rollers (B) disposed at an intermediate position in the secondary scanning direction, and a single first driven roller (C) disposed at a most downstream position in the secondary scanning direction. The pressing force on a trailing edge of the recording sheet ( 10 ) is dividedly released in three steps sequentially and, accordingly, the nipping force and feeding force working on the recording sheet ( 10 ) also vary stepwise. Load fluctuations that occur at the moment the trailing edge of the recording sheet ( 10 ) is released from the nipping force of the first transport roller ( 4 ) also vary stepwise, whereby it is not possible that the feed precision is disturbed in a moment.

TECHNICAL FIELD

The present invention relates to an ink jet printer configured toperform printing by jetting ink while feeding recording paper sheets(including recording media other than paper).

BACKGROUND ART

FIG. 3 is a perspective view showing a conventional ink jet printer andFIG. 4 is a side view of the conventional ink jet printer. In FIGS. 3and 4, a recording sheet fed by a non-illustrated pickup roller isnipped between a transport roller 4′ and a driven roller A′ and thenmoved a predetermined distance toward the left-hand side in the figure(in the secondary scanning direction). Subsequently, an ink head 12′held by a carriage 2′ disposed downstream of the transport roller 4′ inthe secondary scanning direction jets ink against the surface of therecording sheet while reciprocating in the primary scanning directionorthogonal to the secondary scanning direction. While intermittentfeeding of the recording sheet in the secondary scanning direction andmovement of the ink head 12′ in the primary scanning direction arerepeated, an image forming operation continues. In response to detectionof the trailing edge of the recording sheet just short of passingthrough the nip point of the transport roller 4′ by a non-illustratedsheet sensor, the ink head 12′ stops moving in the primary scanningdirection. Finally, an ejection roller 10′ ejects the recording sheetout of the apparatus.

In this case, a blank of a distance X in which an image is not formed isproduced in a trailing edge portion of the recording sheet, the distanceX corresponding to the spacing between the nozzle position of the inkhead 12′ and the nip point of the transport roller 4′. This results in adrawback that the image forming region of the recording sheet is limitedin the trailing edge portion.

In recent years, some printers have been given the capability of formingan image on a recording sheet up to the trailing edge thereof withoutproducing a blank. Mechanical measures devised for such apparatus toimplement that function include: an arrangement configured to lower thesheet feed speed only for image formation on a portion of a recordingsheet adjacent the sheet trailing edge; an arrangement made lesssusceptible to the influence of load fluctuations that occur at themoment the trailing edge of a recording sheet passes through thetransport roller by increasing the sheet nipping pressure of an ejectionroller section; an arrangement provided with a mechanism for suppressingbacklash in driving the transport roller by a pressure produced by aspring; and an arrangement using parts, such as rollers, manufacturedwith an improved machining precision.

These apparatus, however, have a drawback of incurring complication ofimage formation control and increase in the costs of parts because theyare also configured to perform high-quality image formation based mainlyon the dot-jetting control technology. On the other hand, even in thecase where attention is focused on the leading edge portion of arecording sheet under feeding in FIG. 4, load fluctuations occur at themoment the leading edge of the recording sheet rushes to an ejectiondriven roller 7′ which is configured to nip the recording sheet incooperation with the ejection roller 5′ by exerting a pressing force onthe ejection roller 5′, as in the case of the trailing edge portion.Such load fluctuations make the feed distance unstable, which results indegraded image quality such as color irregularity. A conventionalejection driven roller arrangement generally includes a plurality ofsuch ejection driven rollers 7′ arranged in a row in the primaryscanning direction. Accordingly, at the time the recording sheet rushesto the row of these ejection driven rollers 7′, pressing forces of allthe ejection driven rollers 7′ are exerted on the leading edge of therecording sheet at a time, thus greatly affecting the sheet feedprecision. It is needless to say that this phenomenon becomes moreserious as the pressing force of each ejection driven roller 7′increases.

In attempt to prevent irregular image formation at the trailing edgeportion of a recording sheet, there have been proposed an arrangementwherein two types of materials, i.e., a soft material and a hardmaterial, are used for the material of a first driven roller and tworows of such first driven rollers are arranged in the secondary scanningdirection, and an arrangement wherein an auxiliary member for pressingagainst a recording sheet irrespective of the material and shape thereofis disposed downstream of the first driven roller (see patent document 1for example). However, there is neither any mention of an arrangement ofdriven rollers of the same material in the primary scanning direction,nor any description of an arrangement of driven rollers pressing againstejection rollers. There has also been proposed an arrangement for apressure control such as to release or substantially release thepressure of a driven roller pressing against a transport roller halfwaythrough a printing operation (see patent documents 2 and 3 for example).

However, in a conventional ink jet printer which does not utilize thehigh-precision dot control technology, the amount of a feed or move of arecording sheet fluctuates due to fluctuations in the load on the sheetfeed force which occur at the moment the trailing edge of the recordingsheet passes through the transport roller, so that a line deviationoccurs in the secondary scanning direction, thus causing a problem ofdegraded image quality such as color irregularity. Likewise, the sheetfeed precision is affected at the time the leading edge of the recordingsheet rushes to the ejection driven roller, thus resulting in degradedimage quality such as color irregularity.

An object of the present invention is to provide an ink jet printerwhich can ensure improved image quality without incurring complicationof image formation control and increase in cost.

-   Patent Document 1: Japanese Patent Laid-Open Publication No.    H05-186086-   Patent Document 2: Japanese Patent Laid-Open Publication No.    H07-033279-   Patent Document 3: Japanese Patent Laid-Open Publication No.    H11-208923

DISCLOSURE OF THE INVENTION

The present invention includes the following arrangements as means forsolving the foregoing problems.

(1) An ink jet printer comprising: a first transport roller to be drivenin a sheet feed direction in which a recording sheet is to be fed;plural first driven rollers configured to press the recording sheetagainst the first transport roller to nip the recording sheettherebetween; a second transport roller to be driven in the sheet feeddirection at location downstream of the first transport roller in thesheet feed direction; plural second driven rollers configured to pressthe recording sheet against the second transport roller to nip therecording sheet therebetween; and an ink head configured to jet inkagainst the recording sheet while moving in a direction orthogonal tothe sheet feed direction between the first transport roller and thesecond transport roller in the sheet feed direction, the jetting of inkagainst the recording sheet by the ink head being continued even afterpassage of a trailing edge of the recording sheet between the firsttransport roller and the first driven rollers, wherein:

biasing members each configured to exert a biasing force on a respectiveone of the plural first driven rollers in such a direction as to pressthe respective one of the plural first driven roller against the firsttransport roller are disposed at different positions in the directionorthogonal to the sheet feed direction; and

nip points between the first transport roller and the first drivenrollers are located at respective positions in the sheet feed directionin such a manner as to take respective positions in the directionorthogonal to the sheet feed direction which are substantially symmetricwith respect to a center line of the recording sheet parallel with thesheet feed direction.

In this arrangement, the pressure contact points (nip points) betweenthe first transport roller and first driven rollers located upstream ofthe ink head in the sheet feed direction are located at differentpositions in the sheet feed direction in such a manner as to takerespective positions in the direction orthogonal to the sheet feeddirection which are substantially symmetric with respect to a centerline of the recording sheet parallel with the sheet feed direction.Accordingly, during passage of the trailing edge of the recording sheetbetween the first transport roller and the first driven rollers, thepressing force against the recording sheet is released gradually andequally in the direction orthogonal to the sheet feed direction. Forthis reason, the recording sheet can be fed smoothly during the passageof its trailing edge between the first transport roller and the firstdriven rollers. Thus, the condition of an image formed on the trailingedge portion of the recording sheet can be prevented from being degradednotwithstanding the fact that the ink head continues to jet ink againstthe recording sheet during that period, whereby satisfactory imagequality can be ensured.

(2) The biasing forces of the plural biasing members are establishedsuch that a group consisting of a single or plural first driven rollerstaking the same position in the sheet feed direction exerts a totalpressing force on the first transport roller which decreases as thelocation of the group goes downstream in the sheet feed direction.

In this arrangement, a group consisting of a single or plural firstdriven rollers taking the same position in the sheet feed directionexerts a total pressing force on the first transport roller whichdecreases as the location of the group goes downstream in the sheet feeddirection. Accordingly, during passage of the trailing edge of therecording sheet through plural nip points in the sheet feed direction,fluctuations in the load on the recording sheet are reduced gradually.Thus, the recording sheet can be fed more smoothly during the passage ofits trailing edge between the first transport roller and the firstdriven rollers.

(3) The plural second driven rollers are disposed at different positionsin the sheet feed direction and plural second driven rollers arearranged in the direction orthogonal to the sheet feed direction at atleast a most upstream one of the different positions.

In this arrangement, the nip points between the second transport rollerand second driven rollers located downstream of the ink head in thesheet feed direction are located at different positions in the sheetfeed direction. Accordingly, during passage of the leading edge of therecording sheet between the second transport roller and the seconddriven rollers, the pressing force against the recording sheet increasesgradually. Thus, the recording sheet can be prevented from beingsubjected to large load fluctuations during passage of its leading edgebetween the second transport roller and the second driven rollers,whereby the recording sheet can be fed smoothly.

(4) The pressing forces of the plural second driven rollers areestablished such that a most upstream one of groups of second drivenrollers in the sheet feed direction exerts a smallest total pressingforce on the second transport roller, the groups each consisting of asingle or plural second driven rollers taking the same position in thesheet feed direction.

In this arrangement, a most upstream one of groups of second drivenrollers in the sheet feed direction exerts a smallest total pressingforce on the second transport roller, the groups each consisting of asingle or plural second driven rollers taking the same position in thesheet feed direction. Accordingly, during passage of the leading edge ofthe recording sheet through the most upstream one of the nip pointsbetween the second transport roller and the second driven rollers in thesheet feed direction, the recording sheet is prevented from beingsubjected to excessive load fluctuations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an ink jet printer according to anembodiment of the present invention;

FIG. 2 is a side view showing a portion of concern of the ink jetprinter;

FIG. 3 is a perspective view showing a conventional ink jet printer; and

FIG. 4 is a side view showing a portion of concern of the conventionalink jet printer.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a perspective view showing an ink jet printer according to anembodiment of the present invention. FIG. 2 is a side view showing aportion of concern of the ink jet printer. In FIG. 2, a recording sheet10 is fed in the secondary scanning direction toward the left-hand sidein the figure by rotation of a non-illustrated pickup roller driven bymeans of a non-illustrated driving motor and a non-illustrated geartrain.

Subsequently, after a non-illustrated sensor has detected the leadingedge of the recording sheet 10, the recording sheet 10 is nipped betweena first transport roller 4 and first driven rollers A, B and C pressedagainst the first transport roller 4 and then moved a predetermineddistance to reach an image forming position under an ink head 12disposed downstream in the secondary scanning direction. The ink head12, which is held by a carriage 2, jets ink while reciprocating in theprimary scanning direction orthogonal to the secondary scanningdirection, thereby forming an image on the surface of the recordingsheet 10.

Such sheet feed operation in the secondary scanning direction andink-jetting reciprocation of the ink head 12 in the primary scanningdirection are performed intermittently. Finally, the recording sheet 10is ejected out of the apparatus by a second transport roller 5 andsecond driven rollers 71, 72 and 73 disposed downstream of the ink head12.

Two first driven rollers A, two first driven rollers B and one firstdriven roller C are provided for the single first transport roller 4.The first driven rollers A to C are each rotatably supported at one endof a respective one of holders 8. Each of the holders 8 is pivotallysupported at its middle portion on a support shaft 81 of the apparatus.One end of a spring 9 is fixed to the other end of the holder 8. Theholder 8 and the spring 9 constitute a biasing member defined by thepresent invention. The biasing force of the spring 9 acts in such adirection as to press each of the first driven rollers A to C againstthe first transport roller.

A plurality of such biasing members associated with respective of thefirst driven rollers A to C are arranged in the primary scanningdirection. Each of such biasing members may be provided individually fora respective one of the first driven rollers A to C, the total number ofwhich is five, or for a respective one of the pair of first drivenrollers A, the pair of first driven rollers B and the single firstdriven roller C. The two first driven rollers A exert equal biasingforces and, likewise, the two first driven rollers B exert equal biasingforces.

Each biasing member need not necessarily comprise holder 8 and spring 9.The biasing member may be of any other configuration which can exert abiasing force in such a direction as to press the associated one of thefirst driven rollers A to C against the first transport roller 4.

Nip points between the first transport roller 4 and the first drivenrollers A to C are located at different positions in the secondaryscanning direction. The nip points between the first transport roller 4and the first driven rollers A to C are arranged at their respectivepositions in the secondary scanning direction in such a manner as totake respective positions in the primary scanning direction which aresubstantially symmetric with respect to a center line of the recordingsheet parallel with the secondary scanning direction. Specifically, thetwo first driven rollers A are disposed substantially symmetrically withrespect to a center position of the recording sheet lying in the primaryscanning direction at the most upstream position in the secondaryscanning direction. Similarly, the two first driven rollers B aredisposed substantially symmetrically with respect to the center positionof the recording sheet lying in the primary scanning direction at anintermediate position in the secondary scanning direction. Further, thesingle first driven roller C is disposed at the center position of therecording sheet lying in the primary scanning direction at the mostdownstream position in the secondary scanning direction.

Two second driven rollers 71, two second driven rollers 72 and onesecond driven roller 73 are provided for the single second transportroller 5. The second driven rollers 71, 72 and 73 are disposed at theirrespective positions in the secondary scanning direction. The two seconddriven rollers 72 are disposed in the primary scanning direction at themost upstream position.

Nip points between the second transport roller 5 and the second drivenrollers 71 to 73 are located at different positions in the secondaryscanning direction. The nip points between the second transport roller 4and the second driven rollers 71 to 73 are arranged at their respectivepositions in the secondary scanning direction in such a manner as totake respective positions in the primary scanning direction which aresubstantially symmetric with respect to the center line of the recordingsheet parallel with the secondary scanning direction. Specifically, thetwo second driven rollers 72 are disposed substantially symmetricallywith respect to the center position of the recording sheet lying in theprimary scanning direction at the most upstream position in thesecondary scanning direction. Similarly, the two second driven rollers71 are disposed substantially symmetrically with respect to the centerposition of the recording sheet lying in the primary scanning directionat an intermediate position in the secondary scanning direction.Further, the single second driven roller 73 is disposed at the centerposition of the recording sheet lying in the primary scanning directionat the most downstream position in the secondary scanning direction.

The second driven rollers 71 to 73 are biased by means of individualnon-illustrated biasing members in such a direction as to press againstthe second transport roller 5. Each of such biasing members may beprovided individually for a respective one of the second driven rollers71 to 73, the total number of which is five, or for a respective one ofthe pair of second driven rollers 71, the pair of second driven rollers72 and the single second driven roller 73. The two second driven rollers71 exert equal biasing forces and, likewise, the two second drivenrollers 72 exert equal biasing forces.

During a series of feeding operations on the recording sheet 10, first,the first transport roller 4 feeds the recording sheet 10 to an imageformation start position where the ink head 12 forms an image on therecording sheet 10. Thereafter, when the recording sheet 10 is fed apredetermined distance, the leading edge of the recording sheet 10rushes to the nip points between the second transport roller 5 and thesecond driven rollers 72. At that moment, the recording sheet 10 issubjected to some load fluctuations by the pressing force of the seconddriven rollers 72. Since the second driven rollers 72 are formed of twoof the five second driven rollers, load fluctuations on the recordingsheet 10 are relatively small.

In this embodiment, after the leading edge of the recording sheet 10 haspassed between the second driven rollers 72 and the second transportroller 5, the leading edge is caught between the second transport roller5 and the two second driven rollers 71 spaced a distance L1 apart fromthe second driven rollers 72 downstream in the secondary scanningdirection and then, finally, caught between the second transport roller5 and the single second driven roller 73 spaced a distance L2 apart fromthe second driven rollers 72 downstream in the secondary scanningdirection.

Since the recording sheet 10 is caught in three steps sequentially asthe leading edge thereof advances through the group of second drivenrollers 71 to 73 as described above, the recording sheet 10 is subjectedto reduced load fluctuations. For this reason, the precision with whichthe recording sheet 10 is fed is less influenced and, hence, degradationin image quality such as image formation irregularity can hardly occurat the moment the leading edge of the recording sheet 10 advances intothe group of second driven rollers 71 to 73, so that a satisfactoryimage quality is ensured.

On the other hand, as the image forming operation in both the secondaryscanning direction and the primary scanning direction continues, thetrailing edge of the recording sheet 10 reaches the nip points of thefirst transport roller 4. The first driven rollers A to C are arrangedin this order as spaced from each other by a distance Y1 and a distanceY2 in the secondary scanning direction. Each of the first driven rollersA to C is rotatably supported by a respective one of the holders 8 andexerts a pressing force on the first transport roller 4 by means ofspring 9 through the holder 8 pivoting about its pivoting center 81.Also, the first driven rollers A to C are disposed in such a manner thatthe two pairs of the first driven rollers including the first drivenrollers A and the first driven rollers B are arranged symmetrically withrespect to the first driven roller C in the primary scanning direction,as shown in FIG. 1.

With this arrangement, as the image forming operation continues, thetrailing edge of the recording sheet 10 first leaves the two nip pointsbetween the first driven rollers A and the first transport roller 4 and,at that time, the trailing edge portion of the recording sheet 10 isnipped at the nip points between the two first driven rollers B and thefirst transport roller 4 and between the single first driven roller Cand the first transport roller 4. Subsequently, the trailing edge of therecording sheet 10 leaves the nip points between the two first drivenrollers B and the first transport roller 4 and, at that time, thetrailing edge portion of the recording sheet 10 is still nipped at thenip point between the single first driven roller C and the firsttransport roller 4. Finally, the trailing edge of the recording sheet 10is released from the nip point between the first driven roller C and thefirst transport roller 4 and, accordingly, the recording sheet 10 is fedin the secondary scanning direction by means of the second transportroller 5 and the second driven rollers 71 to 73 only.

The image forming operation is further continued until the end positionof the image forming region (about 3 mm apart from the trailing edge ofthe recording sheet 10 in this embodiment) is found to be reached by atrailing edge detection signal from a non-illustrated sheet sensor.

During the above-described image forming operation on a portion of therecording sheet 10 adjacent the trailing edge thereof, the pressingforce on the trailing edge of the recording sheet 10 is dividedlyreleased in three steps sequentially and, accordingly, the nipping forceand feeding force working on the recording sheet 10 also vary stepwise.For this reason, load fluctuations that occur at the moment the trailingedge of the recording sheet 10 is released from the nipping force of thefirst transport roller 4 also vary stepwise, whereby the occurrence ofsuch a phenomenon that the feed precision is disturbed in a moment canbe prevented. Thus, a satisfactory image quality can be ensured.

As a method of further suppressing load fluctuations that occur at themoment the leading edge portion of the recording sheet 10 rushes to thesecond transport roller 5 and at the moment the trailing edge portion ofthe recording sheet 10 is released from the first transport roller 4, itis possible to provide differences in pressing force between the firstdriven rollers A to C and between the second driven rollers 71 to 73thereby to obtain a more smooth variation in feed precision.

The pressing forces of the groups of the first driven rollers A to C areestablished such that a group consisting of a single or plural firstdriven rollers taking the same position in the sheet feed directionexerts a total pressing force on the first transport roller 4 whichdecreases as the location of that group goes downstream in the sheetfeed direction. For example, the pressing forces PA, PB and PC ofrespective of the first driven rollers A, B and C are established tosatisfy the relationship: 2 PA>2 PB>PC.

On the other hand, the pressing forces of the plural second drivenrollers 71 to 73 are established such that a most upstream one of thegroups of second driven rollers in the sheet feed direction exerts asmallest total pressing force on the second transport roller, the groupseach consisting of a single or plural second driven rollers taking thesame position in the sheet feed direction. For example, the pressingforces P71, P72 and P73 of respective of the second driven rollers 71,72 and 73 are established to satisfy the relationship: 2 P72>2 P71>P73.

It is to be noted that the arrangement orders and numbers of the firstdriven rollers A to C and second driven rollers 71 to 73 are not limitedto the foregoing embodiment.

1. An ink jet printer comprising: a first transport roller to be drivenin a sheet feed direction in which a recording sheet is to be fed;plural first driven rollers configured to press the recording sheetagainst the first transport roller to nip the recording sheettherebetween; a second transport roller to be driven in the sheet feeddirection at location downstream of the first transport roller in thesheet feed direction; plural second driven rollers configured to pressthe recording sheet against the second transport roller to nip therecording sheet therebetween; and an ink head configured to jet inkagainst the recording sheet while moving in a direction orthogonal tothe sheet feed direction between the first transport roller and thesecond transport roller in the sheet feed direction, the jetting of inkagainst the recording sheet by the ink head being continued even afterpassage of a trailing edge of the recording sheet between the firsttransport roller and the first driven rollers, wherein: biasing memberseach configured to exert a biasing force on a respective one of theplural first driven rollers in such a direction as to press therespective one of the plural first driven roller against the firsttransport roller are disposed at different positions in the directionorthogonal to the sheet feed direction; and nip points between the firsttransport roller and the first driven rollers are located at respectivepositions in the sheet feed direction in such a manner as to takerespective positions in the direction orthogonal to the sheet feeddirection which are substantially symmetric with respect to a centerline of the recording sheet parallel with the sheet feed direction. 2.The ink jet printer according to claim 1, wherein the biasing forces ofthe plural biasing members are established such that a group consistingof a single or plural first driven rollers taking the same position inthe sheet feed direction exerts a total pressing force on the firsttransport roller which decreases as the location of the group goesdownstream in the sheet feed direction.
 3. The ink jet printer accordingto claim 1, wherein the plural second driven rollers are disposed atdifferent positions in the sheet feed direction and plural second drivenrollers are arranged in the direction orthogonal to the sheet feeddirection at at least a most upstream one of the different positions. 4.The ink jet printer according to claim 3, wherein the pressing forces ofthe plural second driven rollers are established such that a mostupstream one of groups of second driven rollers in the sheet feeddirection exerts a smallest total pressing force on the second transportroller, the groups each consisting of a single or plural second drivenrollers taking the same position in the sheet feed direction.